My parents' cat got SO skinny

I was at my parents' house today and I took a long hard look at Shadow, the family cat and he is SO skinny. He is bony. He is almost 15 years old and was always a very full cat, but now he is lighter than my 8 month old kittens. I feel so sad looking at him knowing that he is nearing the end. Last year, around this time, we had to put down the other family cat and I think since Shadow lost his buddy and I got married and moved out, I think he has been depressed. I don't know what to do for him. I feel so bad. I feel like I drove him to be this way by leaving. I told my mom I wanted to take him when I got married since I was the one who gave him the most attention and it was my bed he slept on all day and all night (with me in it) but she wanted him to stay in her house.

I feel so bad looking at him. It is hard to pet him because he is bony. I want to cry when I see him.